The present invention relates to a process and apparatus for the controlled-pressure casting of molten metals, particularly light alloys of aluminum and magnesium.
Various methods are already known for the casting of bodies molded in molten metal, and in particular in light alloys, within openable metal dies The most common casting processes entail gravity feeding into the closed dies and the creation of the required pressure by means of well-known feedheads, the function whereof is to feed the molten casting of liquid metal by increasing the hydrostatic load on the liquid metal and thus to prevent the formation of cavities and porosities inside the casting during shrinkage in the solidification step.
Another method for casting bodies molded in molten metal is so-called low-pressure casting, which substantially consists in placing the die or mould over a closed furnace connected by means of a tube to the casting cavity defined in the die and in then exerting a pressure on the surface of the metal contained in the furnace so that the liquid metal rises through the tube and feeds the die. The pressure required for casting is maintained for a preset time.
In some casting devices which operate at low pressure, the casting cavity is connected to a source of pressurized gas, generally air, which partially contrasts the pressure exerted on the free surface of the metal in the furnace.
Known methods which provide casting under pressure, especially for light alloys, comprise pressure die-casting, according to which the liquid metal is injected into the die at high pressure; centrifugal casting, according to which the die or dies are subjected to rapid rotation during the casting; and the most recent method, known as "squeeze casting", according to which a given amount of liquid metal is fed into a part of the die and is then compressed by the punch subjected to high pressure.
These casting methods are chosen and used according to the shape and dimensions of the parts to be obtained and according to the alloy to be cast, but in practice they are not always capable of leading to cast bodies which are free from structural defects, such as the presence of internal tensions, micro-porosities and non-uniform compactness during production, which derive mainly from lack of control in the step of solidification of the part.
In particular, for the casting of bodies made of aluminum or magnesium alloy with a complex configuration and with variable-thickness parts, such as light-alloy wheels for motor vehicles and the like, a process for pressure-casting into an openable die, which is capable of providing castings with better mechanical characteristics with respect to those which can be obtained with known methods, in short times and therefore with lower costs, has already been proposed in the European patent application No. 88115342.3 filed on Sep. 19, 1988. The sequence of the steps of execution of this casting process provides: the feeding of the liquid metal, by means of a feedhead or the like, into a casting cavity with an inlet arranged laterally to said cavity and into a distribution chamber which is connected to said casting cavity and is arranged below it, then the completion of the filling of said casting cavity by means of pressure exerted on the metal in said cavity and in said distribution chamber, and then the compression of the solidifying metal by means of a high pressure exerted by the upper part of said casting cavity and from below, if required, by the fixed die. The separation of the components of the die and the extraction of the solidified casting are finally performed.
In practice, this casting process with lateral feeding of the casting cavity entails significant constructive complexity, which is due most of all to the necessary deviations and branchings of the feed duct in order to provide the most complete possible filling of all the regions of the casting cavity. The presence of a lateral feeding channel shaped so as to send the molten metal simultaneously into the casting cavity and into the underlying distribution chamber in fact necessarily entails different values of the filling rate of the various regions of the casting cavity and therefore a distribution of metal which is not always uniform, with consequent values of mechanical resistance of the formed casting which are not always uniformly distributed.
Finally, parts obtained with said process have an insufficient degree of surface finish, so that it is indispensable to perform further finishing operations and remove the protruding stalk or peduncle constituted by the metal which has solidified inside the end of the feedhead and has remained rigidly associated with the formed casting.